The so-called VARTM-technique (Vacuum Assisted Resin Transfer Molding) is a method of manufacturing laminates wherein an underpressure is used for impregnating the various layers of the laminate with resin. The layers are arranged in a single-sided open mould part, on top of which a number of inlet passages are distributed and, at the top, a vacuum foil which is sealed along the edges of the mould part. Then a vacuum is applied in the mould cavity between the foil and the mould part, and the resin is infused or injected from the inlet passages. Following complete or partial hardening of the laminate, the foil is removed and optionally also the inlet passages, and the laminate can be discharged from the mould.
In order to ensure optimal properties of the finished laminate, it is important that all the layers in laminate are completely impregnated with resin and that there are no dry pockets. It is therefore important to distribute and arrange the inlet passages across large parts of the laminate surface in such a manner that the resin supply can be controlled and regulated optimally, which, however, often turns out to be difficult in practice. Simultaneously with ensuring a complete impregnation of all the layers, it is also desirable to be able to perform the injection as quickly as possible.
U.S. Pat. No. 5,328,656 teaches a method of performing vacuum infusion of large shell structures, wherein one or more inlet tubes are conveyed into a cavity in the laminate, following which the cavity is filled from below with a monomer and a hardener. During the filling procedure, the tube is gradually withdrawn to the effect that the outlet end of the tube is continuously just below the flow front. Hereby an even and complete injection into the mould cavity is ensured. However, it is not possible by the above-referenced VARTM-process to move the inlet passages around as they are kept in place by the underpressure and the pressure of the vacuum cloth.
Yet a problem which is associated with the VARTM-process is the large waste of resin involved in the method, since, following the injection, the resin-filled inlet passages either remain on the top face and then partake as a very resin-rich part of the laminate without contributing positively to the properties of the laminate or are removed from the laminate following hardening and must be discarded. By using collapsible inlet bags the passages can be emptied of resin by the pressure of the vacuum foil following finished injection, and the resin waste can thus be reduced. However, this solution presupposes that the vacuum foil is very elastic in order for the inlet passages to open and make room for the resin to enter.
WO 2004/000536 discloses a method of forming a system of temporary inlet passages, wherein a plate consisting of a number of interconnected spacers are arranged in a pattern on top of the vacuum cloth and is covered by one or two further vacuum foils. Hereby the lowermost foil can be sucked up between the spacers to the effect that inlet passages are formed between the material layers and the vacuum foil. However, the described system is complex in that several vacuum foils are used and, likewise, it is required to provide a plate or mat of spacers that specifically fits the shape of the laminate to be produced. Moreover, that method does not solve the problem either of ensuring complete impregnation of the laminate without dry pockets.